1. Field of the Invention
The present invention relates to a filter circuit for smoothing voltage and a power supply unit using the filter circuit.
2. Description of the Related Art
Hitherto, various types of switching power supply units have been proposed and provided for practical use. Many of them are of a type, as shown in FIG. 7 (corresponding to part of FIG. 1 of Japanese Patent Laid-Open No. Hei 11-122911), in which an input DC voltage Vin from a battery (not shown) is switched by switching operation of a switching circuit 102 connected to an input winding of a power conversion transformer 104, an input AC voltage obtained by the switching operation is supplied to the input winding of the power conversion transformer 104, and an output AC voltage obtained as a result of the conversion of the power conversion transformer 104 is output from an output winding of the power conversion transformer 104. A voltage appearing in the output winding in association with the switching operation of the switching circuit 102 is rectified by a rectifier circuit 105, the rectified voltage is converted to an output DC voltage Vout by a smoothing circuit 101, and the smoothed voltage is output.
In a switching power supply unit of this kind, in many cases, the smoothing circuit 101 is configured by serially disposing a smoothing filter circuit 101A for smoothing the output from the rectifier circuit 105, and a noise-reduction filter circuit 101B for reducing noise. The filter circuits 101A and 101B generally include inductors L101 and L102 and capacitors C101 and C102, respectively. To improve the performance of the inductors and to facilitate manufacture of the inductors, generally, magnetic cores 100A and 100B are provided in the inductors L101 and L102, respectively. By providing the magnetic cores 100A and 100B, inductance values increase, so that a ripple voltage of an output of the rectifier circuit 105 and the like can be sufficiently suppressed. Meanwhile, the magnetic cores 100A and 100B have to be disposed separately for the filter circuits 101A and 101B in the two stages, so that the space occupied by the magnetic cores 100A and 100B on the circuit board is large and there is a drawback that it is difficult to reduce the size of the circuit.